The investigations being carried out are aimed at elucidating the interactions occurring between the preimplantation mammalian embryo and the reproductive tract of the female which are required for normal growth in the embryo leading to implantation. Since a critical stage in the preimplantation growth of the embryo appears to be the early blastocyst, the investigations are focused chiefly on the growth requirements of the rabbit embryo at four days post coitum. The growth parameters to be investigated include ribosomal RNA synthesis and processing, membrane transport functions, and membrane synthesis. The studies are to be carried out on embryos maintained in vitro under defined conditions, and the incorporation of radioisotopes into appropriate cellular fractions will be used to measure the embryonic response to variations in these culture conditions.